Assistive manual zeroing visualization

ABSTRACT

A probe having a contact force sensor is inserted into a cardiac chamber and an image of the blood pool is generated. A portion of the blood pool is removed from the image to retain a remaining portion of the blood pool. A determination is made that the distal segment of the probe is within the remaining portion of the blood pool, and responsively to the determination the contact force sensor is manually zeroed.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to cardiac catheterization. More particularly,this invention relates to determination of contact of a catheter withcardiac tissue.

2. Description of the Related Art

Cardiac arrhythmias, such as atrial fibrillation, occur when regions ofcardiac tissue abnormally conduct electric signals to adjacent tissue,thereby disrupting the normal cardiac cycle and causing asynchronousrhythm.

Procedures for treating arrhythmia include surgically disrupting theorigin of the signals causing the arrhythmia, as well as disrupting theconducting pathway for such signals. By selectively ablating cardiactissue by application of energy via a catheter, it is sometimes possibleto cease or modify the propagation of unwanted electrical signals fromone portion of the heart to another. The ablation process destroys theunwanted electrical pathways by formation of non-conducting lesions.

Verification of physical electrode contact with the target tissue isimportant for controlling the delivery of ablation energy. Attempts inthe art to verify electrode contact with the tissue have been extensive,and various techniques have been suggested. For example, U.S. Pat. No.6,695,808 describes apparatus for treating a selected patient tissue ororgan region. A probe has a contact surface that may be urged againstthe region, thereby creating contact pressure. A pressure transducermeasures the contact pressure. This arrangement is said to meet theneeds of procedures in which a medical instrument must be placed in firmbut not excessive contact with an anatomical surface, by providinginformation to the user of the instrument that is indicative of theexistence and magnitude of the contact force.

As another example, U.S. U.S. Pat. No. 6,241,724 describes methods forcreating lesions in body tissue using segmented electrode assemblies. Inone embodiment, an electrode assembly on a catheter carries pressuretransducers, which sense contact with tissue and convey signals to apressure contact module. The module identifies the electrode elementsthat are associated with the pressure transducer signals and directs anenergy generator to convey radiofrequency energy to these elements, andnot to other elements that are in contact only with blood.

A further example is presented in U.S. Pat. No. 6,915,149. This patentdescribes a method for mapping a heart using a catheter having a tipelectrode for measuring local electrical activity. In order to avoidartifacts that may arise from poor tip contact with the tissue, thecontact pressure between the tip and the tissue is measured using apressure sensor to ensure stable contact.

U.S. Patent Application Publication 2007/0100332 describes systems andmethods for assessing electrode-tissue contact for tissue ablation. Anelectromechanical sensor within the catheter shaft generates electricalsignals corresponding to the amount of movement of the electrode withina distal portion of the catheter shaft. An output device receives theelectrical signals for assessing a level of contact between theelectrode and a tissue.

Impedance-based methods for assessing catheter-tissue contact that areknown in the art typically rely on measurement of the magnitude of theimpedance between an electrode on the catheter and a body-surfaceelectrode. When the magnitude is below some threshold, the electrode isconsidered to be in contact with the tissue. This sort of binary contactindication may be unreliable, however, and is sensitive to changes inthe impedance between the body-surface electrode and the skin.

U.S. Patent Application Publication Nos. 2008/0288038 and 2008/0275465,both by Sauarav et al., which are herein incorporated by reference,describe an electrode catheter system, which may comprise an electrodeadapted to apply electric energy. A measurement circuit adapted tomeasure impedance may be implemented between the electrode and ground asthe electrode approaches a target tissue. A processor or processingunits may be implemented to determine a contact condition for the targettissue based at least in part on reactance of the impedance measured bythe measurement circuit. In another embodiment, the contact conditionmay be based on the phase angle of the impedance.

U.S. Patent Application Publication No. 2013/0172875 to Govari et al.,entitled “Contact Assessment Based on Phase Measurement”, which isherein incorporated by reference, describes displaying intra-operativephase determinations of an electrical current passing between theablation electrode and another electrode as an indicator of contactforce between an ablation electrode and target tissue.

Today contact force catheters are commercially available, for examplethe THERMOCOOL® SMARTTOUCH™ Catheter, produced by Biosense Webster,Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765.

SUMMARY OF THE INVENTION

There is provided according to embodiments of the invention a method,which is carried out by inserting a probe having a contact force sensorinto a cavity in a body of a subject, the cavity having a blood pool andan endocardial surface, generating an image of the blood pool, removinga portion of the blood pool from the image to retain a remaining portionof the blood pool thereon, making a determination from the image thatthe distal segment of the probe is within the remaining portion of theblood pool, and responsively to the determination manually zeroing thecontact force sensor.

According to one aspect of the method, the removed portion of the bloodpool is adjacent the endocardial surface.

According to a further aspect of the method, the removed portion of theblood pool is adjacent another probe.

According to an additional aspect of the method, boundaries of theremaining portion of the blood pool are 3 mm from another probe in thecavity and 10 mm from the endocardial surface.

According to a further aspect of the method, boundaries of the remainingportion of the blood pool are 6 mm from another probe in the cavity and13 mm from the endocardial surface.

There is further provided according to embodiments of the invention amethod that is carried out by inserting a probe having a contact forcesensor into a cavity in a body of a subject, the cavity having a bloodpool and an endocardial surface. The method is further carried out bygenerating a first image of the blood pool, generating a second image todefine an excluded region of the blood pool, generating subtractionimages by subtracting the second image from the first image to define azero-qualified region of the blood pool, and while generating thesubtraction images navigating the probe within the cavity, until thedistal portion is within the zero-qualified region.

Another aspect of the method includes making a determination from thesubtraction images that the distal portion is within the zero-qualifiedregion, and responsively to the determination enabling manual zeroing ofthe contact force sensor.

According to still another aspect of the method, a boundary of the otherexcluded region is at least 6 mm from another probe.

According to an additional aspect of the method, the first image and thesubtraction images include the other probe.

There is further provided according to embodiments of the invention anapparatus, including a probe, configured for insertion into a bodycavity having a blood pool, the probe including a contact force sensorfor measuring a force applied to the contact force sensor and locationsensors for detecting a location of the probe in the body cavity, and aprocessor, which is configured to receive a plurality of measurementsfrom the contact force sensor. The processor is operative for generatingan image of the blood pool, removing a portion of the blood pool fromthe image to retain a remaining portion of the blood pool thereon, andpresenting a location of a distal segment of the probe on the image.

According to yet another aspect of the apparatus, the processor isoperative for making a determination from the image that the distalsegment of the probe is within the remaining portion of the blood pool,and responsively to the determination enabling manual zeroing of thecontact force sensor.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the detailed description of the invention, by way of example, whichis to be read in conjunction with the following drawings, wherein likeelements are given like reference numerals, and wherein:

FIG. 1 is a pictorial illustration of a system for performing medicalprocedures in accordance with an embodiment of the invention;

FIG. 2 is a schematic drawing of the distal portion of the cathetershown in FIG. 1 that includes a contact force sensor that can beadjusted in accordance with an embodiment of the invention;

FIG. 3 is a schematic diagram of a cardiac chamber in accordance with anembodiment of the invention;

FIG. 4 is a flow chart of a method of assistive manual contact forcezeroing in a cardiac catheter in accordance with an embodiment of theinvention;

FIG. 5 is a screen display illustrating a phase of the method of FIG. 4in accordance with an embodiment of the invention;

FIG. 6 is a screen display illustrating a phase of the method of FIG. 4in accordance with an embodiment of the invention;

FIG. 7 is a screen display illustrating a phase of the method of FIG. 4in accordance with an embodiment of the invention; and

FIG. 8 is a screen display illustrating a phase of the method of FIG. 4in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the various principles ofthe present invention. It will be apparent to one skilled in the art,however, that not all these details are necessarily needed forpracticing the present invention. In this instance, well-known circuits,control logic, and the details of computer program instructions forconventional algorithms and processes have not been shown in detail inorder not to obscure the general concepts unnecessarily.

In a medical ablation procedure, such as ablation of heart tissue, it isextremely useful to be able to measure the force applied to the tissuewhile the tissue is being ablated. This is because the force applied isa key parameter governing the amount of tissue ablated for a givenablation energy input to the tissue. The ablation is typically providedby a probe comprising an ablation electrode at its distal end. Toaccurately measure a force exerted by the distal tip on the endocardium,the force sensor incorporated into the distal end of the probe istypically calibrated to a “zero level,” also referred to herein as abaseline. The baseline is determined from measurements generated by theforce sensor when the distal tip has minimal contact with any surface(and therefore there is essentially no effective force exerted on thedistal tip). The baseline may be determined using the techniquesdisclosed in U.S. Patent Application Publication No. 2012/0108988 toLudwin et al., which is herein incorporated by reference. Once thebaseline is identified, the measurements from the force sensor can beused to provide a value of the force exerted.

But such force sensors known in the art typically drift. Even if theforce exerted on the sensor is constant, readings from the sensorchange. Such drift may be compensated for by zeroing the sensorperiodically, typically before applying ablation energy. However, thezeroing of the sensor should only be applied if the sensor is notcontacting or in proximity to tissue or other catheters, i.e., thesensor is in a state where the force on it is effectively zero.

The force sensor is assumed to be in a zeroed state if over at least apredetermined interval of time force readings from the sensor change byless than a predetermined force limit. To ensure that the sensor is inthe zeroed state, the probe having the force sensor is typically alsoassumed to change its location during the predetermined time interval bymore than a predetermined location threshold.

Commonly assigned application Ser. No. 14/010,697, entitled “DeterminingNon-Contact State for a Catheter”, whose disclosure is hereinincorporated by reference, teaches how to detect a zeroed state for thesensor, and to calibrate a zero-force point for the force sensor. Inorder to auto-zero the sensor, received signals from the sensor arechecked to detect a situation wherein the sensor is in a first zeroedstate, then in a non-zeroed state (such as if the sensor indicates it istouching tissue), and then in a second zeroed state. Once such asituation is detected, force readings from the second zeroed state maybe used as calibration values that zero the sensor.

The sensor is in a zeroed state where the force on it is effectivelyzero (such a state is typically achieved if the sensor is surrounded byblood in the heart chamber, and is not contacting a heart wall and theprobe is not in proximity to another probe. Changes in proximity betweenprobes may reduce the accuracy of the calibration values referred toabove. In such cases, a probe may be assumed to be in the zeroed stateif, in addition to the force condition described above, a measured valueof the change in proximity to another probe is less than a predeterminedproximity change threshold. In general, there is a high probability ofaccurately auto-zeroing the sensor when the sensor does not contacttissue. In addition, there is an extremely high probability of notauto-zeroing the sensor when the sensor does contact tissue.

Turning now to the drawings, reference is initially made to FIG. 1,which is a pictorial illustration of a system 10 for performing ablativeprocedures on a heart 12 of a living subject, which is constructed andoperative in accordance with a disclosed embodiment of the invention.The system comprises a catheter 14, which is percutaneously inserted byan operator 16 through the patient's vascular system into a chamber orvascular structure of the heart 12. The operator 16, who is typically aphysician, brings the catheter's distal tip 18 into contact with theheart wall at an ablation target site. Optionally, Electrical activationmaps may then be prepared, according to the methods disclosed in U.S.Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat.No. 6,892,091, whose disclosures are herein incorporated by reference.One commercial product embodying elements of the system 10 is availableas the CARTO® 3 System, available from Biosense Webster. This system maybe modified by those skilled in the art to embody the principles of theinvention described herein.

Areas determined to be abnormal, for example by evaluation of theelectrical activation maps, can be ablated by application of thermalenergy, e.g., by passage of radiofrequency electrical current throughwires in the catheter to one or more electrodes at the distal tip 18,which apply the radiofrequency energy to the myocardium. The energy isabsorbed in the tissue, heating it to a point (typically about 50° C.)at which it permanently loses its electrical excitability. Whensuccessful, this procedure creates non-conducting lesions in the cardiactissue, which disrupt the abnormal electrical pathway causing thearrhythmia. The principles of the invention can be applied to differentheart chambers to treat many different cardiac arrhythmias.

The catheter 14 typically comprises a handle 20, having suitablecontrols on the handle to enable the operator 16 to steer, position andorient the distal end of the catheter as desired for the ablation. Toaid the operator 16, the distal portion of the catheter 14 containsposition sensors (not shown) that provide signals to a processor 22,located in a console 24. The processor 22 may fulfill several processingfunctions as described below.

Ablation energy and electrical signals can be conveyed to and from theheart 12 through one or more ablation electrodes 32 located at or nearthe distal tip 18 via cable 34 to the console 24. Pacing signals andother control signals may be conveyed from the console 24 through thecable 34 and the electrodes 32 to the heart 12. Sensing electrodes 33,also connected to the console 24 are disposed between the ablationelectrodes 32 and have connections to the cable 34.

Wire connections 35 link the console 24 with body surface electrodes 30and other components of a positioning sub-system for measuring locationand orientation coordinates of the catheter 14. The processor 22, oranother processor (not shown) may be an element of the positioningsubsystem. The electrodes 32 and the body surface electrodes 30 may beused to measure tissue impedance at the ablation site as taught in U.S.Pat. No. 7,536,218, issued to Govari et al., which is hereinincorporated by reference. A temperature sensor (not shown), typically athermocouple or thermistor, may be mounted on or near each of theelectrodes 32.

The console 24 typically contains one or more ablation power generators25. The catheter 14 may be adapted to conduct ablative energy to theheart using any known ablation technique, e.g., radiofrequency energy,ultrasound energy, and laser-produced light energy. Such methods aredisclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and7,156,816, which are herein incorporated by reference.

In one embodiment, the positioning subsystem comprises a magneticposition tracking arrangement that determines the position andorientation of the catheter 14 by generating magnetic fields in apredefined working volume and sensing these fields at the catheter,using field generating coils 28. The positioning subsystem U.S. Pat. No.7,756,576, which is hereby incorporated by reference, and in theabove-noted U.S. Pat. No. 7,536,218.

As noted above, the catheter 14 is coupled to the console 24, whichenables the operator 16 to observe and regulate the functions of thecatheter 14. Console 24 includes a processor, preferably a computer withappropriate signal processing circuits. The processor is coupled todrive a monitor 29. The signal processing circuits typically receive,amplify, filter and digitize signals from the catheter 14, includingsignals generated by the above-noted sensors and a plurality of locationsensing electrodes (not shown) located distally in the catheter 14. Thedigitized signals are received and used by the console 24 and thepositioning system to compute the position and orientation of thecatheter 14 and to analyze the electrical signals from the electrodes.

During the procedure, contact force between the distal tip 18 orablation electrode 32 and the wall 37 may be measured using a positionsensor in conjunction with the processor 22, or by any of the othertechniques described above for verifying physical electrode contact withthe target tissue.

Typically, the system 10 includes other elements, which are not shown inthe figures for the sake of simplicity. For example, the system 10 mayinclude an electrocardiogram (ECG) monitor, coupled to receive signalsfrom one or more body surface electrodes, so as to provide an ECGsynchronization signal to the console 24. As mentioned above, the system10 typically also includes a reference position sensor, either on anexternally-applied reference patch attached to the exterior of thesubject's body, or on an internally-placed catheter, which is insertedinto the heart 12 maintained in a fixed position relative to the heart12. Conventional pumps and lines for circulating liquids through thecatheter 14 for cooling the ablation site are provided. The system 10may receive image data from an external imaging modality, such as an MRIunit or the like and includes image processors that can be incorporatedin or invoked by the processor 22 for generating and displaying imagesthat are described below.

Reference is now made to FIG. 2, which is a schematic drawing of thedistal portion of catheter 14 showing contact force sensor 39. Thefigure shows and a first position (defined by solid lines) in which thedistal tip 18 is not in contact with the endocardial surface of wall 37.In this position the signal from the sensor 39 can be accurately zeroed(provided no other catheter is nearby) A second position, defined bybroken lines, illustrates a contacting relationship between the distaltip 18 and the wall 37. In the latter condition, the signal from thesensor 39 cannot be accurately zeroed.

Reverting to FIG. 1, operator-assisted contact force zeroing is oftenmore comforting to the operator than the above-noted auto-zeroingtechniques, as he has a degree of control. Confidence on the part of theoperator in the accuracy of the zeroed state is important, as inaccuratecontact force measurements may result in serious complications, such asperforation of the wall and hemopericardium. This is particularly truewhen ablating tissue in right atrium, the thinnest of the cardiacchambers. To assure the operator that the contact force measurement isaccurate, an operator-assisted zeroing visualization procedure isexecuted, e.g., by the processor 22. A map of the heart 12 is displayedon the monitor 29, and regions of the map that qualify for manualzeroing of the catheter become highlighted. The operator navigates thecatheter 14 such that it is located in a highlighted region. As notedabove, the regions qualifying for zeroing in the blood pool are not tooclose (less than 3 mm) to the endocardial surface or to other catheters.Closer proximity than 3 mm may produce system inaccuracies and triggershaft proximity interference mechanisms found in some catheters. Theblood pool may be defined by exploiting the algorithms described in theabove-mentioned application Ser. No. 14/010,697. When more than onecatheter is present, the algorithms may be modified by those skilled inthe art to exclude their neighborhoods from the highlighted areas.Moreover, It is desirable to provide 3 mm safety margins as mentionedabove in the definition of the blood pool and proximity detection inorder to exclude additional regions, which may be problematic due tolimitations in catheter localization accuracy.

The operator-assisted manual zeroing visualization procedure may alertthe operator or even disable his ability to perform manual contact forcezeroing when the catheter is detected, e.g., by the processor 22 inareas that are not suitable for zeroing, i.e., are not highlighted onthe map displayed on the monitor 29.

Reference is now made to FIG. 3, which is a schematic diagram of acardiac chamber 41 illustrating zones varying in suitability for manualcontact force zeroing, in accordance with an embodiment of theinvention. A catheter 43 in the chamber 41 requires contact forcezeroing. The chamber 41 is defined by myocardial wall 45 and anendocardial surface 47. As noted above zeroing is not reliable ifperformed when the catheter is too close to the endocardial surface 47or another catheter 49. The catheter 43 must not be within a firstexclusion zone 51 that extends from the endocardial surface 47 into theblood pool of the chamber 41. As noted above, the exclusion zone 51 istypically 10 mm wide. Moreover, the catheter 43 must not be within asecond exclusion zone 53 about the catheter 49. The exclusion zone 53 istypically 3 mm wide. When the catheter 43 is not within the exclusionzones 51, 53 it is possible to manually zero the contact force sensor.However it is preferable to provide additional safety zones 55, 57 asbuffers about the exclusion zones 51, 53, respectively. The zones 55, 57are typically 3 mm thick. A careful operator will not manually zero thecontact force sensor when the catheter 43 is within the zones 55, 57,but will require that the catheter 43 be in a region 59 of the bloodpool that is not within any of the zones 51, 53, 55, 57. The safeboundaries of the region 59 are thus 13 mm from the endocardial surface47 and 6 mm from the catheter 49.

Reference is now made to FIG. 4, which is a flow chart of a method ofassistive manual contact force zeroing in a cardiac catheter, inaccordance with an embodiment of the invention. The process steps areshown in a particular linear sequence in FIG. 4 for clarity ofpresentation. However, it will be evident that many of them can beperformed in parallel, asynchronously, or in different orders. Thoseskilled in the art will also appreciate that a process couldalternatively be represented as a number of interrelated states orevents, e.g., in a state diagram. Moreover, not all illustrated processsteps may be required to implement the process.

At initial step 61 catheterization of a cardiac chamber is accomplishedconventionally. A contact force catheter and optionally other cathetersare introduced into a cardiac chamber.

Next, at step 63, a definition of the blood pool of the cardiac chamberis displayed as a first image.

Next, at step 65, The blood pool is redrawn to exclude a first region ofthe blood pool adjacent the endocardial surface of the cardiac chamber,referred to herein as a first excluded region. Typically, the firstexcluded region is about 10 mm away from any tissue due to contractionand expansion of the heart - - - . Furthermore, each catheter within thechamber other than the contact force catheter is surrounded by arespective spherical proximity zone, which constitutes a second excludedregion. Steps 63, 65 may be accomplished using the procedures describedin the above-mentioned application Ser. No. 14/010,697. A second imagemay be generated in which the first excluded region and the secondincluded regions are highlighted.

Next, at step 67, the first excluded region and the second exclusionregions defined on the second image in step 65 are subtracted from thefirst image that was produced in step 63, using standard imageprocessing routines. A subtraction image is generated. The portion ofthe blood pool that remains on the subtraction image is referred toherein as a zero-qualified region, because it is suitable for manuallyzeroing the contact force sensor.

Referring again to FIG. 4, next, at step 69, the catheter is navigatedby the operator and new images of the distal portion of the catheter andthe blood pool are generated. The zero-qualified region may behighlighted to assist the operator.

Next, at decision step 71, it is determined by evaluation of the newimages if the catheter is in the zero-qualified region that wasestablished at step 67. If the determination is negative, then controlreturns to step 69 and the catheter is repositioned.

If the determination at decision step 71 is affirmative then controlproceeds to final step 73. The operator zeroes the contact force sensor,and the procedure ends.

Reference is now made to FIG. 5, which is a screen display 75 obtainedafter completion of initial step 61 (FIG. 4) in accordance with anembodiment of the invention. The screen display 75 shows blood pool 77of a cardiac chamber 79 in which is found an ablation catheter 81 havinga contact force sensor 83 at its distal end. A mapping catheter 85 isalso present in the cardiac chamber 79.

Reference is now made to FIG. 6, which is a screen display 87 obtainedafter completion of step 63 in accordance with an embodiment of theinvention. The initial definition of the blood pool is highlighted anddemarcated by a solid line 89. Portions of the blood pool 77 in a zone91 external to the line 89 define the above-described first excludedregion. Such portions are not suitable for contact force zeroing, asthey are too close to the endocardial surface. It will be appreciatedthat while the screen display 87 is an exemplary 2-dimensionalprojection of a 3-dimensional object, the display can be varied, torepresent many views and projections in order to enable the operator toappreciate the location of the catheter anywhere within the interior ofthe cardiac chamber 79.

Reference is now made to FIG. 7, which is a screen display 93 obtainedafter completion of step 65, in accordance with an embodiment of theinvention. A spherical zone, which appears roughly as a circle 95 in the2-dimensional projection of FIG. 7 demarcates the above-described secondexcluded region about the catheter 85. Although not shown in FIG. 7,respective exclusion regions of this sort would be demarcated about allother catheters found in the cardiac chamber 79 (other than the contactforce catheter 81).

Reference is now made to FIG. 8, which is a screen display 97 of asubtraction image obtained after completion of step 67 in accordancewith an embodiment of the invention. The remaining portion of the bloodpool 77, outlined by solid line 99 represents the zero-qualified regionin which contact force zeroing of the contact force sensor 83 can beaccomplished with confidence.

The procedure shown in FIG. 4 is represented Listing 1 by pseudo-code,which can be implemented on an image processor.

Listing 1

Create the chamber volume and draw all catheters in CatheterList

BloodPoolVolume=Find Blood pool( )

Mark BloodPoolVolume volume with color;

For each Catheter in CatheterList

If Catheter not ContactForceCatheter then

-   -   Mark catheter exclusion zone with VolumetricSphere

End If

Next Catheter

Calculate ManualZeroSuggestion as

BloodPoolVolume—Union of VolumetricSpheres

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. A method, comprising the steps of: inserting a probe having a distalsegment and a contact force sensor into a cavity in a body of a subject,the cavity having a blood pool and an endocardial surface; generating animage of the blood pool; removing a portion of the blood pool from theimage to retain a remaining portion of the blood pool thereon; making adetermination from the image that the distal segment of the probe iswithin the remaining portion of the blood pool; and responsively to thedetermination manually zeroing the contact force sensor.
 2. The methodaccording to claim 1, wherein the removed portion of the blood pool isadjacent the endocardial surface.
 3. The method according to claim 1,wherein the removed portion of the blood pool is adjacent another probe.4. The method according to claim 1, wherein boundaries of the remainingportion of the blood pool are 3 mm from another probe in the cavity and10 mm from the endocardial surface.
 5. The method according to claim 1,wherein boundaries of the remaining portion of the blood pool are 6 mmfrom another probe in the cavity and 13 mm from the endocardial surface.6. A method, comprising the steps of: inserting a probe having a distalportion and a contact force sensor into a cavity in a body of a subject,the cavity having a blood pool and an endocardial surface; generating afirst image of the blood pool; generating a second image to define anexcluded region of the blood pool; generating subtraction images bysubtracting the second image from the first image to define azero-qualified region of the blood pool; and while generating thesubtraction images navigating the probe within the cavity; until thedistal portion is within the zero-qualified region.
 7. The methodaccording to claim 6, further comprising the steps of: making adetermination from the subtraction images that the distal portion iswithin the zero-qualified region; and responsively to the determinationenabling manual zeroing of the contact force sensor.
 8. The methodaccording to claim 6, wherein margins of the first image are at least 10mm from the endocardial surface.
 9. The method according to claim 6,wherein margins of the first image are at least 13 mm from theendocardial surface.
 10. The method according to claim 6, furthercomprising the steps of: inserting another probe into the cavity,including on the second image another excluded region about the otherprobe.
 11. The method according to claim 10, wherein a boundary of theother excluded region is at least 3 mm from another probe.
 12. Themethod according to claim 10, wherein a boundary of the other excludedregion is at least 6 mm from another probe.
 13. The method according toclaim 10, wherein the first image and the subtraction images include theother probe. 14-21. (canceled)